FUNCTIONAL-ANALYSIS OF THE PHOSPHOPROTEIN P-II (GLNB GENE-PRODUCT) INTHE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC-7942

The P-II protein (glnB gene product) in the cyanobacterium Synechococc us sp. strain PCC 7942 signals the cellular N status by being phosphor ylated or dephosphorylated at a seryl residue. Here we show that the P -II-modifying system responds to the activity of ammonium assimilation via the glutamine synthetase-glutamate synthase pathway and to the st ate of CO2 fixation. To identify possible functions of P-II in this mi croorganism, a P-II-deficient mutant was created and its general pheno type was characterized. The analysis shows that the P-II protein inter feres with the regulation of enzymes required for nitrogen assimilatio n, although ammonium repression is still detectable in the P-II-defici ent mutant. We suggest that the phosphorylation and dephosphorylation of P-II are part of a complex signal transduction network involved in global nitrogen control in cyanobacteria. In this regulatory process, P-II might be involved in mediating the tight coordination between car bon and nitrogen assimilation.